Creators of Spark Core have also created a guide to help answer backer questions and aid future crowdsource campaign starters. As the title suggests the guide runs through potential curveballs that can cause manufacturing delays. One notable reason is adding features to your project suggested by your backers. While causing a delay you are ultimately delivering a better product to your supporters. There are many more other good bits of advice in their guide and the benefit to readers of this site is gearing the article to electronic hardware manufacturing. Enjoy.